One of the difficulties commonly encountered when using a ladder is finding an appropriately located section of ground, or other surface, on which to place the ladder so that it can be safely used. Far too often the surface at the desired location is inappropriately sloped, or one leg of the ladder tends to sink into the surface more readily than one or more of the other legs. Unfortunately, when the user of a ladder is faced with an undesirable surface the tendency is to employ some makeshift "propping device" to shore-up one or more of the legs in order to achieve, and hopefully maintain, at least a modicum of plumb to the ladder.
Poorly selected "propping devices" often result in either damage or injury to the ladder and/or its user. These harsh consequences have heretofore prompted the development of ladders having extendible legs which incorporate some locking arrangement. The intended result to be achieved by the use of extendible legs is to effect facile adjustment to the length of each leg and thereby accommodate a wide variety of unfavorable surfaces which might reasonably be expected to be encountered. However, there always appears to be room for improvement.
Perhaps one of the most successful of the prior art extension leg locking devices is disclosed in my prior U.S. Pat. No. 3,016,103. That patent discloses a leg extension apparatus which utilizes a ratchet-type locking mechanism to allow the legs of the ladder to extend downwardly to the surface, or surfaces, on which the ladder is to be placed, and then be able to lock the extended legs in the position chosen so that the ladder will be relatively plumb, irrespective of the surface on which the ladder is resting. However, when the locking mechanism is released the ratchet permits the legs to retract for convenient storage. Unfortunately, the aforesaid ratchet type locking mechanism is rather complicated, and as a result is more difficult to use, is slightly more prone to malfunction, and is, therefore, somewhat more difficult to manufacture. Moreover, the relative complexity of the ratchet type locking mechanism also serves to make it somewhat unreliable.
The tendency toward occasional unreliability occurs because the connection between the fixed legs and the extension legs is effected by a ratchet-and-tooth configuration. As such, the ratchet must align directly with a tooth in order to effect the best possible gripping interaction therebetween. Occasionally a ratchet may not fully engage the recess between two successive teeth, but may, instead, catch on the edge, or apex, of a tooth, giving the impression that it is securely seated, but being unexpectedly able to disengage as soon as weight is applied, or shortly thereafter. When such an arrangement does slip, the ratchet member will generally engage within that recess between the next two successive teeth. This prevents the ladder from totally collapsing. Nevertheless, the ladder may jar the user sufficiently to cause a loss of balance that could result in a fall.
Other endeavors to provide locking mechanisms which permit selective adjustability for the extendible legs of a ladder allow the extension legs to protract freely when the ladder is not subjected to any load and which preclude the extension legs from retracting when the ladder must support the combined weight of the ladder and the user. That is, when the ladder is resting on firmly-planted extension legs the entire weight of the ladder and the user serve to actuate the locking mechanism and thereby prevent any further movement of the extension legs. This device is not ideal because it possesses an inherent tendency to release its locking effect when the weight applied to the ladder is partially, or even momentarily, reduced beyond a critical value. Accordingly, if a ladder having the latter described mechanism is bumped or jarred, or the user loses his balance while on the ladder, the load can be momentarily reduced to the point that the locking mechanism may release, with disastrous results.